This section provides background information related to the present disclosure which is not necessarily prior art.
Friction stir welding tools are well known from the prior art, wherein they allow butt joint connections to be realized in such a way that the two workpieces are initially laid next to each other along two edges and then the friction stir welding tool, while being rotationally driven around the rotation axis, is moved along the area of the edges lying next to each other across the workpieces, which is made possible by the fact that the rotation of the tool causes the plasticizing of the material of the workpieces in this area. In the process the engagement section of the pin initially engages with the edges of the workpieces, while the first and the second contact surfaces engage with the workpieces on their top side and bottom side respectively.
Depending on whether the contact surfaces or the shoulder elements on which they are provided are also rotationally driven, friction energy may additionally be introduced into the workpieces via the surfaces.
In this regard, it is known from the prior art to rotationally drive, independently of the pin, in particular the top shoulder provided on the side of the drive end of the engagement section, so that the first contact surface can rotate in the opposite direction or in the same direction as the pin.
Throughout this document, whenever reference is made hereafter to a “top” or a “bottom” shoulder element, it shall always be assumed that the friction stir welding tool is arranged such that the rotary drive for the pin is mounted above the pin. The “top shoulder” is then arranged closer to the rotary drive than an optionally provided “bottom” shoulder and, in particular, the “top” shoulder is arranged closer to the rotary drive than the engagement section. Thus, the terms “top” and “bottom” are to be understood as indicating a position relative to the rotary drive, with a top element being arranged closer to the rotary drive than an optionally also provided bottom element. The terms “top” and “bottom” do not necessarily indicate the spatial location, either.
Although, from a construction perspective, it has proven to be comparatively easy to rotationally drive, independently of the pin, the shoulder arranged at the top and adjacent to the rotary drive for the pin with the contact surface provided thereon, this is a very difficult task for a contact surface which is arranged on the bottom end or free end of the pin spaced apart from the rotary drive and thus on the side of the engagement section facing away from the rotary drive. In such arrangements, only hollow shaft assemblies may be used for the pin, and these are complicated to manufacture and, in addition, they often do not satisfy the strength requirements.
Indeed, it is desirable that the torque, which is transmitted from the pin to a shoulder element, which is provided on the side of the engagement section facing away from the rotary drive, can be limited, so that the friction energy introduced via the bottom contact surface can also be limited. It is also desirable that a device which permits such limiting of the friction energy introduced via the surface has a simple construction and functions reliably.